Nickel hydride batteries, lithium secondary batteries, and other nonaqueous electrolyte secondary batteries that can be charged and discharged have been of growing importance recently as on-board power sources for vehicles and as power sources for personal computers and handheld devices. In particular, lithium secondary batteries, which are lightweight and provide a high energy density, are well-suited for use as high-power on-board energy sources for vehicles.
This type of lithium secondary battery is typically built by housing, together with an electrolyte, an electrode assembly that is formed of a positive electrode and a negative electrode stacked on top of one another with a separator therebetween, within a case. Electrode constructions in such electrode assemblies that are known to the art include stacked electrode assemblies in a form obtained by stacking together a plurality of flat plate-like electrode assemblies, and coiled electrode assemblies in a form obtained by spirally coiling a continuous sheet-like electrode assembly. By adopting such constructions, the reaction surface area between the positive and negative electrodes is enlarged, enabling the energy density and power to be increased.
The separator here is typically made of resin, and has the role of electrically insulating between the positive electrode and the negative electrode and also the role of holding the nonaqueous electrolyte solution. In addition to these roles, to ensure the safety of the battery and the machine in which the battery has been installed, the separator also has the function of, when the battery interior overheats and reaches a given temperature region (typically the softening point or melting point of the resin), softening and thereby interrupting the charge carrier conduction path (referred to herein as “shutdown”). It is also desirable for this separator, because it is exposed to a reducing potential near the negative electrode and is exposed to a reducing potential near the positive electrode, to have a resistance to these environments (particularly an oxidation resistance to the oxidizing atmosphere at the positive electrode).
It has thus been proposed that such a separator have, on one or both faces of a base material composed of a conventional resin, a heat-resistant layer containing both an inorganic filler having heat resistance and a binder (see, for example, Patent Documents 1 and 2). For example, by providing such a heat-resistant layer on at least the surface of the separator on the side thereof facing the positive electrode, it is possible to prevent oxidative deterioration due to the positive electrode. Also, providing the separator with a heat-resistant layer enables the separator to maintain electrical insulation between the positive and negative electrodes even after shutdown, making it possible to prevent a leakage current from arising.
At the same time, over the past few years, progress has been made in achieving higher power in large secondary batteries adapted for use as, e.g., power sources for driving vehicles (such as lithium secondary batteries for hybrid automobiles). For instance, when a short circuit arises at the battery interior due to, for example, the inadvertent admixture of metal impurities, it is conceivable that a sudden rise in the battery temperature occurs. In such a case, the temperature near the short-circuit point on the surface of the negative electrode is even higher, and may reach several hundred degrees Celsius (e.g., 300° C. or more). Hence, in such large secondary batteries, the prevention of separator meltdown due to internal shorting by providing a heat-resistant layer on at least the surface of the separator on the side facing the negative electrode is also being investigated.